Development and Evaluation of an Improved Apparatus for Measuring the Emissivity at High Temperatures

An improved apparatus for measuring the spectral directional emissivity in the wavelength range between 1 µm and 20 µm at temperatures up to 2400 K is presented in this paper. As a heating unit an inductor is used to warm up the specimen, as well as the blackbody reference to the specified temperatures. The heating unit is placed in a double-walled vacuum vessel. A defined temperature, as well as a homogenous temperature distribution of the whole surrounding is ensured by a heat transfer fluid flowing through the gap of the double-walled vessel. Additionally, the surrounding is coated with a high-emitting paint and serves as blackbody-like surrounding to ensure defined boundary conditions. For measuring the spectral directional emissivity at different emission angles, a movable mirror is installed in front of the specimen, which can be adjusted by a rotatable arrangement guiding the emitted radiation into the attached FTIR-spectrometer. The setup of the emissivity measurement apparatus (EMMA) and the measurement procedure are introduced, and the derived measurement results are presented. For evaluating the apparatus, measurements were performed on different materials. The determined emissivities agree well with values published in literature within the derived relative uncertainties below 4% for most wavelengths.

Broadband directional control of thermal emission

Controlling the directionality of emitted far-field thermal radiation is a fundamental challenge. Photonic strategies enable angular selectivity of thermal emission over narrow bandwidths, but thermal radiation is a broadband phenomenon. The ability to constrain emitted thermal radiation to fixed narrow angular ranges over broad bandwidths is an important, but lacking, capability. We introduce gradient epsilon-near-zero (ENZ) materials that enable broad-spectrum directional control of thermal emission. We demonstrate two emitters consisting of multiple oxides that exhibit high (>0.7, >0.6) directional emissivity (60° to 75°, 70° to 85°) in the p-polarization for a range of wavelengths (10.0 to 14.3 micrometers, 7.7 to 11.5 micrometers). This broadband directional emission enables meaningful radiative heat transfer primarily in the high emissivity directions. Decoupling the conventional limitations on angular and spectral response improves performance for applications such as thermal camouflaging, solar heating, radiative cooling, and waste heat recovery.

The Time-series Mid-Infrared Data Simulation for High-temporal Resolution Geostationary Satellite

<p>Geostationary remote sensing satellite can provide time-series mid-infrared (MIR) data at regional scale, which plays a significant role in many applications such as environmental monitoring, fire detection and temporal change of surface parameters. Therefore more geostationary remote sensing satellite missions for earth observation are carried out and focused on directional and high-temporal resolution. Given the complex nature of the data to be expected from these missions, it is essential for a thorough preparation, which can be accomplished by simulating the image data before the actual launch. The simulation can include the top-of-atmosphere (TOA) radiance data as well as all major process parameters such as land surface temperature/emissivity and atmospheric parameters. It can be used to evaluate the capabilities of target satellite observing the earth and optimize the system according to the further analysis. In addition, the development of the data simulation will provide a considerable support for the algorithms of quantitative application.</p><p>This work addressed a method for simulating the time-series mid-infrared data of geostationary satellite based on radiative transfer model. The simulation procedure, including directional emissivity, time-series LST, time-series atmospheric parameter, sensor performance, can be shown as follows. Firstly, an empirical Bidirectional Reflectance Distribution Function (BRDF) model, i.e., the Minnaert’s model, is introduced to describe the non-Lambertian reflective behavior of land surface. Then, the directional emissivity can be calculated based on the Kirchhoff’s law with the John Hopkins University (JHU) Spectral Library as the prior knowledge. Secondly, a semi-empirical Diurnal Temperature Cycle (DTC) model with six parameters (Göttsche, F. M., and Olesen, F. S., 2001) is used to simulate the time-series LST with the interval of 15min. Thirdly, the atmospheric profiles of pressure, temperature, relative humidity (RH), and geo-potential (GP) at 0.5° latitude/longitude spatial resolutions for 8 UTC times per day provided by European Centre for Medium-Range Weather Forecasts (ECMWF) are used for atmospheric parameters. A temporal interpolation method is proposed to obtain the time-series atmospheric parameters from the ECMWF 3-hour profile. Then, the MIR spectral radiance at the top of atmosphere can be simulated by MIR radiative transfer equation with the aid of MODTRAN 5 code. Finally, by convoluting the sensor’s spectral response function, the radiance received by the sensor can be got against the instrument noise. The results show that the time-series mid-infrared data for geostationary satellite of different surface types at any angle can be well simulated using the proposed method. More comparative analysis with the geostationary satellites, such as METEOSAT, GEOS, FENGYUN, GMS etc., will be done in the future work.</p>

c-Si PV cells emissivity characterization at low operating temperatures for efficiency management

Efficiency is a critical parameter for a solar cell to be successful and is closely related to the working temperature of the cell. In turn, the temperature can be related to the infrared emissivity, the parameter that governs the thermal radiative properties of a body. In particular, the importance of infrared emissivity in a solar cell is essential in passive cooling applications, where controlled radiative thermal losses could let the cell operate at lower temperatures, the range where they present higher efficiency. In this presentation, the emissivity of c-Si solar cells in the low temperature range (around 50 ºC) is discussed. Traditionally, it has been determined by indirect reflectivity measurements at ambient temperature and extrapolated to working temperatures, but here, a direct measurement is proposed. For an accurate value the measurements are performed in the high accuracy radiometer of the University of the Basque Country, which allows spectral directional emissivity measurements as a function of temperature.

Evaluation of Six Directional Canopy Emissivity Models in the Thermal Infrared Using Emissivity Measurements

Land surface temperature (LST) is a fundamental physical quantity in a range of different studies, for example in climatological analyses and surface–atmosphere heat flux assessments, especially in heterogeneous and complex surfaces such as vegetated canopies. To obtain accurate LST values, it is important to measure accurately the land surface emissivity (LSE) in the thermal infrared spectrum. In the past decades, different directional emissivity canopy models have been proposed. This paper evaluates six radiative transfer models (FR97, Mod3, Rmod3, 4SAIL, REN15, and CE-P models) through a comparison with in situ emissivity measurements performed using the temperature-emissivity separation (TES) method. The evaluation is done using a single set of rose plants over two different soils with very different spectral behavior. First, using an organic soil, the measurements were done for seven different observation angles, from 0° to 60° in steps of 10°, and for six different values of leaf area index (LAI). Taking into account all LAIs, the bias (and root mean square error, RMSE) obtained were 0.003 (±0.006), −0.004 (±0.005), −0.009 (±0.011), 0.005 (±0.007), 0.004 (±0.007), and 0.005 (±0.007) for FR97, Mod3, Rmod3, 4SAIL, REN 15, and CE-P models, respectively. Second, using an inorganic soil, the measurements were done for six different LAIs but for two different observation angles: 0° and 55°. The bias (and RMSE) obtained were 0.012 (±0.014), 0.004 (±0.007), −0.020 (±0.035), 0.016 (±0.017), 0.013 (±0.015), 0.013 (±0.015) and for FR97, Mod3, Rmod3, 4SAIL, REN15, and CE-P models, respectively. Overall, the Mod3 model appears as the best model in comparison to the TES emissivity reference measurements.